1. Field of the Invention
The present invention relates to a semiconductor package that mounts a plurality of semiconductor chips in combination, and merges them by installing wiring between the chips to exchange data, that is, to a semiconductor device assembled in a multi-chip package.
2. Description of Related Art
Recently, an increasing number of multi-chip packages (called xe2x80x9cMCPsxe2x80x9dfrom now on) have been used in electronic equipment such as mobile phones requiring miniaturization and versatility, as semiconductor products meeting demands of equipment manufacturers. The MCP consists of one package including combinations of LSIs such as logic and memory, digital and analog, and flash memory and SRAM LSIs, which are stacked and have wiring installed between the chips by wire bonding.
FIG. 13 is a block diagram showing a schematic internal configuration of a conventional multi-chip package including two-chips (semiconductor devices) combined together. In FIG. 13, the reference numeral 1100 designates an MCP, and reference numerals 1110 and 1120 each designate a chip constituting a semiconductor device mounted on the MCP 1100. Although the two chips usually have different types of functions, it is not unlikely that they belong to the same type. In either case, they are assembled into the multi-chip structure to transfer data in one direction from one chip to the other, or to exchange data between them.
In the first chip 1110, the reference numeral 500 designates an internal circuit, reference numerals 410 and 420 each designate an input buffer for the internal circuit 500, and 430 and 440 each designate an output buffer. Reference numerals 101-104 designate pads that are formed on the chip, and are connected to the input terminals of the input buffers 410 and 420 and the output terminals of the output buffers 430 and 440, respectively. Reference numerals 105 each designate a pad for one of other input/output terminals of the internal circuit 500 (not shown for the sake of simplicity). In the second chip 1120, the reference numeral 501 designates an internal circuit, 411 designates an input buffer for the internal circuit 501, and 441 designates an output buffer. The reference numeral 201 designates a pad formed on the chip 1120 to be connected to the input terminal of the input buffer 411. Reference numerals 202, 203 and 205 each designate a pad to be connected to one of the input/output terminals of the internal circuit 501 (not shown for the sake of simplicity) . Both the chips 1110 and 1120 have a configuration for exchanging data. Specifically, the pads 101 and 204 and the pads 104 and 201 are each interconnected by wires 701 and 702. Thus, the output of the buffer 440 of the chip 1110 drives the chip 1120 via the pads 104 and 201, and the output of the buffer 441 of the chip 1120 is supplied to the internal circuit 500 of the chip 1110 via the pads 204 and 101. Reference numerals 601-606 designate external terminals of the MCP 1100 used for the chip 1110, which are connected to the pads 102, 103 and 105 via the wires 703-708. Reference numerals 611-616 designate external terminals of the MCP 1100 used for the chip 1120, which are connected to the pads 202, 203 and 205 via wires 723-728.
Generally, the pads, that is, the input/output terminals and output terminals of the semiconductor devices mounted on the MCP, fall into two types: the first type of pad is used for the external input/output terminals and output terminals after assembly (used in a state in which the MCP is installed into electronic equipment); and the second type of pad is used for the input and output only between the semiconductor devices assembled into the MCP.
As for the second type, Japanese patent application No. 2001-294539 applied by the assignee of the present invention discloses it. It discloses a configuration that controls the output driving power of the output buffer 440 by using a control signal 150 as shown in FIG. 13. FIG. 14 shows an example of the circuit configuration of the output buffer 440.
In FIG. 14, the reference numeral 443 designates a normally used driver, and 444 designates a power adjusting driver. The output buffer 440 is supplied with a signal from the internal circuit 500 of FIG. 13 as its input signal 160, and its output appears at the output pad 104. At a wafer test, the control signal 150 is placed at a xe2x80x9cHxe2x80x9d (high) level to enable the power adjusting driver 444. Thus, the output buffer 440 can increase its driving power so that it can drive a large load capacitance of a tester. In addition, in the normally used state after the assembly, the power adjusting driver 444 is disabled by placing the control signal 150 at a xe2x80x9cLxe2x80x9d (low) level. Thus, the output buffer 440 can reduce its driving power after assembly, so that it drives only the another semiconductor device mounted on the MCP, that is, only the chip 1120 in the example of FIG. 13. By thus switching the driving power in accordance with the usage condition, the output buffer 440 can drive the wiring between the chips in the MCP 1100 with smaller driving power in the normally used mode, thereby curbing the generation of drive noise affecting the operation, and limiting the increase in current consumption at the operation.
With the foregoing configuration, the conventional MCP has the following problems. First, consider the state in which the output buffer must drive only the another semiconductor device installed in the MCP after the assembly. In this case, the output buffer 440 carries out its driving through the normally used driver 443 only. In the course of this, the normally used driver 443 must always drive the drain capacitance of a P-channel transistor 446 and N-channel transistor 447 constituting the power adjusting driver 444. In other words, even though the output buffer 440 is controlled such that its driving power is reduced to drive only the another semiconductor device, it must simultaneously drive with its small driving power the drain capacitance of the power adjusting driver 444 in the off state. Accordingly, the size of the normally used driver 443 must be determined considering the drain capacitance of the power adjusting driver 444 in the off state. This offers a problem of increasing the current consumption by that amount.
Second, as for the semiconductor devices to be installed in the MCP, and a structure provided for driving the another semiconductor device in the conventional technique to implement the MCP, the following problem arises. It is impossible for the output buffer 440 to restrict its size to a small one because the normally used driver 443 requires the driving power for driving the drain capacitance of the power adjusting driver 444 in the off state. For this reason, countermeasures against the noise generation and current consumption at the operation are limited, which hinders the optimization of the MCP product.
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a semiconductor device that can supply the driving power required for a wafer test, and drive the another semiconductor device installed in the MCP while restricting the current consumption and preventing the drive noise adversely affecting the normal operation.
According to a first aspect of the present invention, there is provide a semiconductor device including: a first pad to be connected to another semiconductor device; a second pad for making a probing connection in a wafer test; a first buffer connected to the first pad for driving the another semiconductor device; and a second buffer driven by the first buffer, for driving a load capacitance of a tester connected to the second pad by driving power greater than that of the first buffer, the second buffer having its active/inactive state controlled by a control signal. Thus, it can drive the another semiconductor device through the first buffer with the smaller driving power after assembly. In the wafer test, it can drive the load capacitance of the tester through the second buffer with the greater driving power. Consequently, it offers an advantage of being able to suppress noise generation and current consumption during the operation. In particular, it can reduce the power consumption in the buffer in the normal mode.
According to a second aspect of the present invention, there is provided a multi-chip package including at least a first chip and a second chip, wherein the first chip has the same configuration as the semiconductor device of the first aspect, and the second buffer is controlled by the control signal to an active state during the wafer test, and to an inactive state during a normal operation of the multi-chip package.